1. Field of the Invention
This invention relates to a process for the removal of noxious nitrogen oxides NOx from combustion flue gases which contain sulfur oxides (SO.sub.2, SO.sub.3) as well as nitrogen oxides. Nitrogen oxides in combustion flue gas are catalytically reduced to nitrogen and water at a relatively low temperature in the presence of sulfur oxides, especially sulfur trioxide, in a reaction with ammonia, which is added to the flue gas.
2. Description of the Prior Art
Several processes for the removal of nitrogen oxides from combustion flue gas have been developed in recent years. The most promising process is a catalytic process in which a NOx-containing gas is put in contact with a catalyst and NOx is reduced to nitrogen and the water in a reaction with ammonia. Usually the reaction is carried out at 200.degree. .about. 450.degree. C. In cases where the combustion flue gas contains rather high concentrations of sulfur oxides, there are several problems with the process.
The most serious problem is the accumulation of ammonium sulfate [(NH.sub.4).sub.2 SO.sub.4 ] and ammonium bisulfate (NH.sub.4 HSO.sub.4) (hereafter referred to collectively as ammonium sulfates) on the surface of the catalyst employed, which necessarily results in a decrease in and some times a loss of ability of the catalyst. From an economical point of view the reaction is preferrably carried out at lower temperatures. However the deposition of ammonium sulfates, is more serious at lower temperatures, especially those below 330.degree. C., because the vapor pressure of ammonium sulfates is low.
The formation of ammonium sulfates is expressed as follows. EQU NH.sub.3 + SO.sub.3 + H.sub.2 O .fwdarw. NH.sub.4 HSO.sub.4 ( 1) EQU 2nh.sub.3 + so.sub.3 + h.sub.2 o .fwdarw. (nh.sub.4).sub.2 so.sub.4 ( 2)
the concentration of sulfur trioxide is said to be 1 .about. 10% of that of the total sulfur oxides in a flue gas. Therefore, if the concentration of SOx is 1,000 ppm, the concentration of SO.sub.3 is 10 .about. 100 ppm.
In the catalytic reduction of NOx at lower temperatures if ammonia is added to a flue gas, the formation of ammonium sulfates are inevitable. The deposition of ammonium sulfates readily occurs in the micropores of the catalysts, because of the phenomenon called "capillary condensation", which occurs in those micropores. For the reasons stated above, the catalytic reduction of NOx with NH.sub.3 is usually carried out at temperatures above 350.degree. C.
A process for the regeneration of catalyst which have become inactive because of the deposition of ammonium sulfates has also been proposed under Japanise laid-open of patent application No 51-3366 (1976) "Process for removing of nitrogen oxides" where the catalysts are heated to high temperatures, 400.degree. .about. 600.degree. C., and the deposited ammonium sulfates are vaporised. There are several problems associated with this process.
(1) In the regeneration process a high concentration of ammonium sulfates is discharged into the atmosphere, which presents the possibility of secondary pollution. Therefore, equipment to remove the ammonium sulfates must be installed.
(2) In the NOx reduction process, a catalyst which comprises an alumina carrier and active components, for example V, Cu, and Fe is usually employed. When the catalyst is heated to 400.degree. .about. 600.degree. C. in the presence of ammonium sulfates, alumina is converted to aluminum sulfate and/or ammonium aluminum sulfate, resulting in a decrease in catalytic activity.
(3) Ammonia, which is the NOx reducing agent, is lost in the formation of ammonium sulfates. The lost ammonia accounts for about 10% of the ammonia utilized in the reduction of NOx.